Coatings Removal,waterjets

1. Demonstration, validation and certification of forced
pulsed waterjet technique for the removal of coatings
from aircraft/aerospace components
M. Vijay, A. Tieu, W. Yan and B. Daniels
VLN Advanced Technologies, Ottawa, ON, Canada
J. Randolph, F. Laguines, D. Crawford, C. Pessetto and J. Merrill
Engineering & Software System Solutions (ES3), Clearfield, Utah, USA
R. Eybel, K. Bucknor and M. Game
Messier-Dowty (Safran Group), Ajax, ON, Canada
Abstract
Extensive work is in progress in the laboratory to investigate the feasibility of using
forced pulsed waterjet (FPWJ) technique for removing a variety of coatings (Chrome,
HVOF, plasma) from aircraft/aerospace components. Airworthiness, that is, certification
of the technique as safe for aerospace applications requires: (1) demonstration that the
FPWJ does indeed remove the coatings without damaging the substrate material and (2)
validation that it will not alter the properties of the components (dimensions,
compressive stresses, etc.). While demonstration stage has been completed successfully,
further work continues to validate FPWJ as airworthy and green technique for the
removal of coatings.
1 INTRODUCTION
The collaborative work reported in this paper has been in progress for more than five
years (1, 2 & 3). The major goal of the work is to seek certification from regulatory
agencies in aviation industry to use the patented forced pulsed waterjet (FPWJ) technique
for the removal of Chrome, HVOF (high-velocity oxy-fuel) and other types of coatings
from aircraft/aerospace components. A number of studies have indicated that hexavalent
Chromium, which occurs through oxidation of lower valence Chromium compounds
during plating, causes, among other health risks, lung cancer. Therefore, OSHA has
issued a final rule on permissible exposure limit (PEL) of 0.5-Fg/m3 (4, 5, 6 & 7).
Implementation of such stringent requirement would not only make Chrome plating
prohibitively expensive, but also would substantially increase turnaround time (TAT) of
processing of components. In order to circumvent this problem, several alternative
coatings, particularly hard and highly wear resistant HVOF coatings, consisting of
Tungsten Carbide (WC), Cobalt (Co) and other thermal spray particles, have been
proposed (5, 6, 7 & 8). However, as explained by Tieu, et al. (2), HVOF coating's
intrinsic durability also makes it difficult to be stripped for inspection and recoating.
Current stripping methods involve multiple processes such as grinding, wet chemical
baths (which may require repeat dips) and grit blasting, depending on the type and

2. thickness of coating. Ultra-high pressure (UHP) continuous waterjet (CWJ), which is
used extensively for stripping conventional thermal-spray metallic coatings, has been
found to be inefficient for removing HVOF coatings. Preliminary investigations
conducted at the request of several aerospace/aircraft companies have shown that FPWJ
has a great potential for removing HVOF and other coatings (1). The final
implementation of the FPWJ technique for stripping of these coatings on a regular basis
(both commercial and military), however, will depend on its certification as safe
(airworthy) by both national and international regulatory agencies (OEM, FAA, USAF,
etc.). Certification requires demonstration and validation (Dem/Val) of the technique,
which are briefly described in the following Section.
2 PROCEDURES FOR CERTIFICATION
Simply stated, the procedures are:
• Demonstration of the technique that it does function as claimed by the provider
(OEMs or government organizations) of the technique;
• Qualification, which involves evaluating the technique for (a) performance
(effectiveness compared to other existing or proposed techniques), (b) safety
(airworthiness, which is legal) and (c) environmental compliance {meeting the
requirements of organizations such as, EPA (Environmental Protection Agency of
USA)} and (d) operator friendly.
In practice the certification procedure could be quite complex, starting from (9):
• Design generation (customer requirements and negotiations with regulatory
agency);
• Release of product definition, which involves analysis of design;
• Substantiation of design by calculation and tests, which may involve design
changes;
• Substantiation of changes made after tests and before production to meet
certification standards;
• Verification and approval by the customer;
• Incorporation of changes into the prototype;
• Certification by the provider and the regulatory agency.
In the case of FPWJ technique for stripping the coatings from aerospace/aircraft
components, satisfying the following requirements is quite essential.
• No significant alteration in surface texture (specifications by organizations such as
ASME B46.1) – to be evaluated using surface roughness measurements,
metallography and SEM/EDX analysis of the surface of the substrate;
• Mass loss expectation – no measurable dimensional changes of the parts (diametric
and weight measurements before and after stripping, etc.);
• Does not alter surface compressive stresses of the parts, that is, does not cause
premature failure of parts processed with FPWJ by fatigue – to be established by
fatigue testing of standard bars (conducted by the end-user);
• Does not cause failure of the parts by hydrogen embrittlement – to be tested by the
end-user;
• Does not alter surface and material defects, permitting routine inspections;
• Reproducibility of performance (consistent surface finish, etc.);

3. • Reliability of the equipment and environmental compliance.
In this paper, highlights of the work done to date are described in the following Sections.
It must be stated in passing that names of corporations or government agencies for whom
or, with whom the work was conducted are not disclosed (due to confidentiality
agreements) in this paper (except ES3 and Messier-Dowty).
3 TECHNICAL BACKGROUND
The method used for generating FPWJ by modulating a continuous stream of water has
been reported in several publications (10). As described thoroughly by Bai, et al. (11)
and by Vijay, et al. (12), high-frequency pulses of water are generated by modulating a
CWJ by locating a probe (microtip) inside the nozzle driven by an ultrasonic transducer-
generator assembly. When a pulse of water impinges on a target, the pressure of impact
is the waterhammer pressure, which is considerably higher than the normal stagnation
pressure. For example, if the operating pressure of the pump were 69-MPa, the impact
pressure of a pulse would be of the order of 550-MPa. Although the duration of each
impact is quite short (.1-Fs), the combination of high frequency (20-kHz) and high
impact pressure makes the material to yield quite readily. In simple terms, this means
that if a certain job can be done with a CWJ, the same job can be done at much lower
pressures using FPWJ. As a consequence, the pulsed waterjet machines are compact in
size, portable, safe to operate, environmentally compatible, user friendly, and as affirmed
by Messier-Dowty, is green viable technology (2). Furthermore, these machines offer
another attractive feature. That is, they can be operated in FPWJ mode or, CWJ mode
simply by turning on or off the ultrasonic generator. Thus, the machines can be used for
the removal of hard or soft coatings (10).
4 EXPERIMENTAL WORK
4.1 Demonstration
As stated above, the purpose of demonstration is to prove to the end-users the
effectiveness and potential of FPWJ for stripping a variety of coatings from
aircraft/aerospace components without damaging the substrates. From this perspective,
some of the interesting and most challenging applications are illustrated in Figs. 1 to 7.
All tests were conducted on an X-Y gantry or, using a five-axis Kawasaki robot or, using
a hand-held gun to illustrate all operating possibilities. The highlights are:
Figure 1: These simple tests were conducted for an airline corporation. FPWJ removed
the 1.27-mm thick HVOF (WC-Co) coating quite readily at 103.5-MPa. The surface
finish achieved in tests #11 & 12 were considered to be excellent by the airline.
Figure 2: The results depicted in Fig. 2 were obtained for a landing gear company. A
single-orifice nozzle (d = 1.372-mm) at P = 69-MPa was used to remove the HVOF
coating from the landing gear pin. The surface finish (profile) was considered to be
excellent by the client (Fig. 2C).
Figure 3: Removing epoxy coating from landing gear components is illustrated in Fig. 3.
The coating was effectively removed to Cadmium base at a pressure of only 69-MPa.
The surface finish, depicted in Figs. 3B, C, E and F, was considered to be excellent by
the client.

4. Figure 4: In this figure removing HVOF coating from an aircraft frame is depicted. The
magnitudes of As and Es were respectively were the order of 0.4-m2/hr and 85.0-
kWhr/m2 at a pressure of only 69-MPa.
Figure 5: A hand-held gun consisting of a dual-orifice (d = 1.02-mm of each orifice)
rotating nozzle at 69-MPa was used for stripping hard coatings (e.g., aluminized epoxy
and varnish) from the outer surfaces of the propeller gear box and housing.
Figure 6: To remove several types of coatings from the complex parts shown in Fig. 6
was quite challenging. Soft coatings shown in Fig. 6A, were removed with the FPWJ at
pressures # 34.5-MPa. HVOF coatings on the bore of the PT support (Fig. 6C) and the
balance piston wheel teeth (Fig. 6E, F) were removed (including the grooves) at
pressures of the order of 90-MPa, the durations ranging from 30 to 150-s. Plasma coating
on the inner surface of the compressor was removed at 83-MPa in less than 90-s. The
surface finishes of all the parts were assessed to be excellent by the aircraft corporation.
Figure 7: Removal of thermal barrier coating from the combustion chamber outer liner is
illustrated in Fig. 7. It is obvious from the close-up view of the chamber that FPWJ at a
pressure of the order of 100-MPa was quite effective in removing the coating without
damaging the surface.
In summary, the tests under “demonstration” did confirm that FPWJ is indeed quite
effective for removing several types of coatings from complex aircraft/aerospace parts.
4.2 Qualification
The first series of tests to qualify the FPWJ process was conducted in collaboration with
Messier-Dowty (2 & 3). The samples were landing gear pins (Fig. 8) coated with 0.38-
mm thick WC-Co-Cr (as sprayed). The observations from this very preliminary and
limited study were:
• The surface finish of the substrate was excellent (see Fig. 8).
• The surface residual stress measurements made with XRD (X-ray diffraction)
showed that cyclic loading of FPWJ did not alter the compressive material surface
stresses of the pin.
Following these highly encouraging observations, further work was conducted in
collaboration with ES3. Typical results achieved in this collaborative project are depicted
in Figs. 9 to 15. Relevant data and observations are summarized in Table 1.
5 DISCUSSION
Qualitatively, close-up photographs are useful for recording visual observations of
the substrate (damage, finish, flash rust, etc.). Although excessive damage (deep pitting
etc.) disqualifies the process of stripping, certain degree of erosion (which needs to be
quantified by microscopic measurements and metallography) is necessary for creating
good surface profile. On some coupons flash rust did occur, which can be eliminated by
adding rust inhibitors to water (in the case of HVOF coating, the rust would not be
problem). The term “excellent” stated in Table 1 was based on visual observation of the
coupons after stripping with the FPWJ. The photographs also assist, to some extent, to
understand the mechanism of removal of the coatings from the substrate material.

5. Although a detailed description is beyond the scope of this paper, the dominant
mechanism appears to be peeling (flaking) for Chrome and, erosion in the case of HVOF
(that is, removing particles from the substrate). To summarize, visual observation of the
surface of the coupon after conducting a test at a particular set of operating parameters
was useful in assessing the quality of surface finish obtained (see Fig. 14).
Quantitative assessment of the effectiveness of FPWJ for stripping was based on
measuring before and after stripping: (1) surface roughness, (2) dimensions of the
coupons and (3) mass of coupons. The term “excellent” in Table 1, based on the
measurements of Ra, dimensions and masses of the coupons before and after stripping,
implies that FPWJ would meet the “surface texture” criterion required for qualification
(see Fig. 14). However, from the standpoint of productivity and energy consumption, the
magnitudes of As and Es are also quite important. The following observations are
noteworthy:
• As anticipated, depending upon the type of coating, the magnitudes of As and Es
vary significantly.
• For hard Chrome on 4340, the values of As and Es were better for the 1.372-mm
orifice compared to the 1.626-mm orifice. For Chrome on 300M, on the other hand,
the reverse seems to be the case. The magnitude of As increased from 0.61 to 4.87-
m2/hr (by a factor of eight) when the orifice diameter was increased to 1.626-mm. It
is not clear if this variation is due to the variation in the method of plating the
coupons. Further work is obviously required to confirm this effect.
• The stripping rates of HVOF from 4340 steel (as sprayed or ground) were observed
to be higher than the rates of stripping of HVOF from 300M. It appears that while
300M was sprayed with WC-Co using the JP8000 system, 4340 was sprayed with
WC-Co-Cr using the DJ2600 system. It is not clear whether this difference in
composition and particle size distribution or, the method of coating contributes to
this difference in performance. It is also quite likely that the adhesion process of
particles in HVOF coating is not well understood, and may be the reason for
performance variation. Finally, the difference in performance could be due to the
differences in microstructural material matrices of 4340 and 300M steels.
Understanding these factors would make it possible to enhance the performance of
stripping with FPWJ.
• Another interesting observation in the case of 4340-HVOF is increasing the
thickness from 0.127 to 0.254-mm did not decrease the stripping rate (it remained
constant at 0.362-m2/hr). This observation suggests that adhesion (bond) strength,
rather than the thickness, is probably more important from the standpoint of
stripping.
• In the case of HVOF on 300M, the effect of thickness on As is not clear. When the
thickness increased from 0.076-mm to 0.216-mm, the stripping rate decreased from
0.121 to 0.06-m2/hr. However, for the 0.4445-mm thick coating, stripping rate
increased to 0.093-m2/hr. A plausible explanation is that the process of coating thick
layers may be different than coating thin layers.
• The magnitudes of Es varied significantly in stripping the coupons, the minimum &
maximum being respectively 13.6 to 1,035-kWhr/m2, the goal being to keep it as
low as possible. There are several steps to accomplish this goal. As the stripping
rate is a function of the width of coating removed per pass and traverse speed of
FPWJ, it is possible to increase both by increasing the pressure for a given nozzle
diameter. Other possibilities are employing single-orifice oscillating or, dual-orifice
rotating nozzles.

6. • Surface finish and dimensional stability (that is, no significant changes in the
outside diameters of the coupons) were considered to be excellent. Additionally,
weight measurements of stripped coupons showed that material loss due to FPWJ
stripping process was negligible.
• It should be noted from Fig. 15 that CWJ at the same operating parameters as FPWJ
was ineffective in stripping the HVOF coating from the 300M coupon.
6 CONCLUSIONS
Basically, the main conclusions from the continuing investigations are:
• All demonstrative tests have clearly shown that FPWJ is very effective in removing
several types of coatings from a variety of complex aircraft/aerospace components.
• Tests conducted to date to affirm qualification of the FPWJ for stripping are highly
promising.
• Stripping results (finish etc.) were highly uniform and reproducible.
• Mass loss and dimensional changes caused by the stripping process were within
acceptable levels.
• Performance, as measured by removal rate and specific energy values, are
significantly better than the values achieved by other processing techniques
(chemical dipping, grinding, etc.)
• FPWJ process is environmentally and user friendly as it only uses water (green
viable technology).
• More extensive work is required to receive certification from regulatory agencies
(prior to submitting this paper, cursory tests on coupons for fatigue and hydrogen
embrittlement evaluation of the FPWJ stripping process have been completed.
These results will be available by the end of this year).
7 ACKNOWLEDGMENTS
The work reported in this paper would not have been possible without the continuing
support from aircraft/aerospace and government agencies. Authors from ES3 and VLN
are thankful to USAF for providing partial funding for the project under the SBIR
program. VLN also acknowledges partial funding and technical support received from
Messier-Dowty. Authors are also thankful to Mr. W. Bloom, VLN Tech, for managing
the project.
8 REFERENCES
(1) Vijay, M.M., W. Yan, B. Ren, A. Tieu and B. Daniels, “Removal of hard metallic
and non-metallic aerospace coatings with high-frequency forced pulsed waterjet
machine,” Proc. 18th International Conference on Water Jetting, BHR Group
Limited, Cranfield, Bedfordshire, England, MK43 0AJ, 2006, pp. 367-379.
(2) Tieu, A., W. Yan and M. Vijay, M.M., “Considerations in the use of pulsed
waterjet techniques for the removal of HVOF coatings,” Paper 3-G, Proc. 2007
American WJTA Conference and Expo, Waterjet Technology Association, St.
Louis, MO 63101-1434, USA.
(3) Randolph, J., M. M. Vijay, F. Laguines and K. Bucknor, “Development of forced
pulse water strip of Tungsten carbide HVOF coatings and Chrome plating on

7. aircraft, landing gear and propeller components,” SERDP/ESTCP Workshop –
Surface Finishing and Repair Issues for Sustaining New Military Aircraft, Tempe,
AZ., USA, February 26-28, 2008.
(4) Anon, “OSHA’s final rule for hexavalent Chromium,” Spear Consulting, Magnolia,
Texas, USA.
(5) Starwell, B.D., P.M. Natishan, I.L. Singer, K.O. Legg, J.D. Schell and J.P. Sauer,
“Replacement of Chromium electroplating using HVOF thermal spray coatings,”
HCAT, March 1998.
(6) Anon, “Replacement of Chromium electroplating on landing gear components
using HVOF thermal spray coatings,” ESTCP (Environmental Security Technology
Certification Program) Cost and Performance Report, PP-9608, U.S. Department of
Defence, USA., May 2004.
(7) Anon, “Chrome plating alternatives: thermal spray, electroless plating and others,”
A Thintry Market Study, Thintry Inc., New York, USA, 2005.
(8) Wondoren, van. M., “Landing gear overhaul with HVOF coatings,” Private
Communication, KLM Engineering & Maintenance.
(9) Anon, “Airworthiness Overview,” Report by Aerospace Industries Association of
Canada, Ottawa, ON, Canada.
(10) Vijay, M.M., “Fundamentals and Applications of Cavitating and Forced Pulsed
Waterjet Technologies,” Technical Note, VLN Advanced Technologies, Ottawa,
Canada (can be obtained by request: mvijay@vln-tech.com).
(11) Bai, C., S. Chandra, B. Daniels, B. Ren, W. Yan, A. Tieu and M. Vijay, “Abrasive-
entrained high-frequency pulsed waterjet: basic study and applications,” Proc. 18th
International Conference on Water Jetting, BHR Group Limited, Cranfield,
Bedfordshire, England, MK43 0AJ, pp. 325-335.
(12) Vijay, M.M., A. Tieu, W. Yan and B. Daniels, “Method and apparatus for prepping
surfaces with high-frequency forced pulsed waterjet,” Patent pending (US Patent
Application, 089114555US, July, 2008).
9 NOMENCLATURE
As: Removal rate of the coating, m2/hr
d: Orifice diameter, mm
E s: Specific energy, kWhr/m2
P: Pump pressure, MPa
Ra: RMS value of surface roughness, Fm
Sd: Standoff distance, mm
Vtr: Traverspeed of the nozzle, mm/min
Tc: Thickness of coating, mm

8. Table 1. Summary of relevant experimental data
Surface finish: EXCELLENT
Figure # P (MPa) Sd (mm) Vtr (mm/min) Remarks
9 96.6 . 100 0 d = 1.016
Chrome- .100 Exposed for 60-s. No
steel damage.
10 86.2 76.2 127.0 d = 1.626
Chrome on Tc = 0.076-0.127
4340 steel 1.93 # Ra # 2.39
As = 0.604, Es = 110.8
Chrome on 96.6 101.6 254.0 d = 1.372
4340 steel Tc: same as above
As = 1.217, Es = 46.6
11 86.2 76.2 1,016.0 d = 1.626
Chrome on Tc = 0.076-0.127
300M steel 0.91 # Ra # 1.22
As = 4.87, Es = 13.64
Chrome on 96.6 101.6 127.0 d = 1.372
300M steel Tc: Same as above
1.02 # Ra # 1.75
As = 0.61, Es = 92.70
12 103.5 146.0 76.2 d = 1.372
HVOF on Tc = 0.076-0.127
4340 steel 2.56 # Ra # 3.45
As = 0.362, Es = 172.6
HVOF on 103.5 146.0 76.2 Increased thickness
4340 steel d = 1.372
Tc = 0.203-0.254
As = 0.362, Es = 172.6
13A 103.5 146.0 25.4 d = 1.372
HVOF on Tc = 0.076-0.127
300M steel 2.11 # Ra # 2.49
As = 0.121, Es = 517.7
13B 103.5 146.0 12.7 d = 1.372
HVOF on Tc = 0.216
300M steel 2.84 # Ra # 3.76
(as sprayed) As = 0.06, Es = 1035.4
13C 103.5 146.0 12.7 3.56 # Ra # 3.86
HVOF on Same results as 13B,
300M steel indicating good
(as sprayed) reproducibility.
13D 103.5 146.0 19.05 d = 1.372
HVOF on Tc = 0.4445
300M steel 2.24 # Ra # 2.39
(as sprayed) As = 0.093, Es = 673.4

9. #11
A
2 3 4 5 6 7
#12 1 8
B
Substrate: 300M
Fig. 1. Removal of 1.27-mm thick 52-54-HRC
HVOF: WC-Co-Cr
HVOF (WC-Co) coating from steel Th ickness: 0.08-0.13-m m
substrate at 103.5-MPa at various Polished to R a = 4
traverse speeds (tests conducted for C: Close-up view of
a US airline). C Test #8.
Fig. 2. Removing HVOF coating
from highly-polished landing
gear pin.
A D
E
F
B
C
A, B, C: Hub coated with
epoxy on Cadmium layer
D, E, F: Views of the shaft
before & after removing
epoxy coating.
Fig. 3. Removing epoxy coat on Cadmium base of landing gear components.

10. B
A
Fig. 5. Stripping of hard coatings
Fig. 4. Removing HVOF coating (aluminized epoxy and varnish)
(specification not known) from from propeller gear box and
aircraft frame. housing.
A B
Elbow Suction Line
C D
Close-up
HVOF
PT Support
E F
HVOF Removed
Balance Piston Wheel
G
H
Removed
Plasma
Coat
Compressor Inner Case
Fig. 6. Removing several types of coatings/sealants from a variety of
aircraft parts.

11. Combustion
Removed
Chamber
Cl
os
e-
up
Thermal
Spray
Fig. 7. Removing thermal spray barrier coating from the inside
surface of a combustion chamber.
Surface Finish
HVOF
Removed
Fig. 8. Removal of 0.381-mm thick HVOF coating (WC-Co-Cr)
from a tapered landing gear pin (300M steel, HRc 53-55).
Conducted in collaboration with Messier-Dowty.
(A) (B)
Fig. 9. Removal of 0.24 to 0.38-mm Chrome from cylinders
(rejected landing gear parts). (A) before exposure to FPWJ, (B)
surface finish after removal. Conducted in collaboration with
ES3.
(A) 1 2 3 4 5 6
Fig. 10. Stripping 0.076 to
0.127-mm thick Chrome
from 4340 steel cylindrical
(B) Fully Stripped
coupons. The handwritten
numbers in (B) are the Ra
values measured before and
(C) after removing the coating.
Pressure = 86-MPa
Conducted with ES3.
Close-up
View

12. Surface Roughness Values
Close-up Views
Fig. 11. Stripping 0.076 to 0.127-mm thick Chrome from 300M steel
cylindrical coupons. Ra values are indicated.
Pressure = 86-MPa. Conducted with ES3.
Fig. 12. A close-up view of the 4340-steel cylindrical coupon from which
0.076 to 0.127-mm thick HVOF (WC-Co or, WC-Co-Cr) coating was
removed at 103.5-MPa. Conducted with ES3.
Fig. 13A. Removal of 0.076 to 0.127-mm thick HVOF from 300M.
Conducted with ES3.

13. Fig. 13B. Same as in Fig. 13A, except thickness = 0.203-mm.
Fig. 13C. Same as in Fig. 13B, except for different method of coating.
Fig. 13D. Same as in Fig. 13C, except thickness = 0.444-mm.

14. Fig. 14. Surface texture characteristics observed with a
microscope. Magnification = 25.
Bare Bare
Metal Metal
HVOF Coating on 300M- 9
Bare CWJ
Metal 60s
FPWJ
CWJ
30-s
45-s
15-s
60-s
60-s
Close-up View
Fig. 15. Comparison of the performance of FPWJ with
the CWJ for removing HVOF coating at the same
operating conditions (d = 1.372-mm, P = 103.5-MPa).